diff --git a/src/gui/add_csv_window.cpp b/src/gui/add_csv_window.cpp
index 893ae061..4bfb0894 100644
--- a/src/gui/add_csv_window.cpp
+++ b/src/gui/add_csv_window.cpp
@@ -115,7 +115,7 @@ std::vector<std::string> AddCSVWindow::readCSVRow(const std::string& row) {
   return fields;
 }
 
-bool AddCSVWindow::readCSV(std::ifstream& in, std::vector<String> headers_out, std::vector<std::vector<ScriptValueP>>& table_out) {
+bool AddCSVWindow::readCSV(std::ifstream& in, std::vector<String>& headers_out, std::vector<std::vector<ScriptValueP>>& table_out) {
   // Get the rows
   vector<std::string> raw_rows;
   std::string raw_row;
@@ -140,7 +140,7 @@ bool AddCSVWindow::readCSV(std::ifstream& in, std::vector<String> headers_out, s
       row = row + "\n";
     }
   }
-  if (rows.size() == 0) {
+  if (rows.empty()) {
     queue_message(MESSAGE_ERROR, _ERROR_1_("import empty file", _("CSV / TSV")));
     return false;
   }
@@ -183,7 +183,7 @@ void AddCSVWindow::onOk(wxCommandEvent&) {
   // Check for missing fields
   String missing_fields;
   check_table_headers(set->game, headers, _("CSV / TSV"), missing_fields);
-  if (missing_fields.size() > 0) {
+  if (!missing_fields.empty()) {
     queue_message(MESSAGE_WARNING, _ERROR_2_("import missing fields", _("CSV / TSV"), missing_fields));
   }
   // Produce cards from the table
diff --git a/src/gui/add_csv_window.hpp b/src/gui/add_csv_window.hpp
index dfe70c20..1fd75985 100644
--- a/src/gui/add_csv_window.hpp
+++ b/src/gui/add_csv_window.hpp
@@ -28,7 +28,7 @@ protected:
 	SetP            set;
 	char            separator;
 
-	bool readCSV(std::ifstream& in, std::vector<String> headers_out, std::vector<std::vector<ScriptValueP>>& table_out);
+	bool readCSV(std::ifstream& in, std::vector<String>& headers_out, std::vector<std::vector<ScriptValueP>>& table_out);
 	std::vector<std::string> readCSVRow(const std::string& row);
 	
 	void onSeparatorTypeChange(wxCommandEvent&);
diff --git a/src/gui/add_json_window.cpp b/src/gui/add_json_window.cpp
index bc08d9eb..78e37f7a 100644
--- a/src/gui/add_json_window.cpp
+++ b/src/gui/add_json_window.cpp
@@ -63,7 +63,7 @@ AddJSONWindow::AddJSONWindow(Window* parent, const SetP& set, bool sizer)
   }
 }
 
-static ScriptValueP json_to_script(boost::json::value jv) {
+static ScriptValueP json_to_script(boost::json::value& jv) {
   if (jv == nullptr) return script_nil;
   else if (jv.is_null()) return script_nil;
   else if (jv.is_bool()) return to_script(jv.get_bool());
@@ -237,7 +237,7 @@ void AddJSONWindow::onOk(wxCommandEvent&) {
   // Check for missing fields
   String missing_fields;
   check_table_headers(set->game, headers, _("JSON"), missing_fields);
-  if (missing_fields.size() > 0) {
+  if (!missing_fields.empty()) {
     queue_message(MESSAGE_WARNING, _ERROR_2_("import missing fields", _("JSON"), missing_fields));
   }
   // Produce cards from the table
diff --git a/src/script/functions/construction_helper.hpp b/src/script/functions/construction_helper.hpp
index c637fe49..22574911 100644
--- a/src/script/functions/construction_helper.hpp
+++ b/src/script/functions/construction_helper.hpp
@@ -83,7 +83,7 @@ static bool set_builtin_container(GameP& game, CardP& card, ScriptValueP& value,
 }
 
 static bool check_table_headers(GameP& game, std::vector<String>& headers, const String& file_extension, String& missing_fields_out) {
-  if (headers.size() == 0) {
+  if (headers.empty()) {
     queue_message(MESSAGE_ERROR, _("Empty headers given"));
     return false;
   }